Electroactive polymers capable of performing a deflection in response to a voltage is well known technology, like the compliant electrodes described in U.S. Pat. No. 6,376,971, being positioned in contact with a polymer in such a way, that when applying a potential difference between the electrodes, the electric field arising across the polymer thickness contracts the electrodes against each other, thereby deflecting the polymer. Since the electrodes are of a substantially rigid material, they must be made textured in order to make them compliant.
The electrodes are described as having an ‘in the plane’ or ‘out of the plane’ compliance. In U.S. Pat. No. 6,376,971 the out of the plane compliant electrodes may be provided by stretching a polymer more than it will normally be able to stretch during actuation, and a layer of stiff material is deposited on the stretched polymer surface. For example, the stiff material may be a polymer that is cured while the electroactive polymer is stretched. After curing, the electroactive polymer is relaxed, and the structure buckles to provide a textured surface. The thickness of the stiff material may be altered to provide texturing on any scale, including submicrometer levels. Alternatively, textured surfaces may be produced by reactive ion etching (RIE). By way of example, RIE may be performed on a pre-strained polymer comprising silicon with an RIE gas comprising 90 percent carbon tetrafluoride and 10 percent oxygen to form a surface with wave troughs and crests of 4 to 5 micrometers in depth. As another alternative, the electrodes may be adhered to a surface of the polymer. Electrodes adhering to the polymer are preferably compliant and conform to the changing shape of the polymer. Textured electrodes may provide compliance in more than one direction. A rough textured electrode may provide compliance in orthogonal planar directions.
Also in U.S. Pat. No. 6,376,971 there is disclosed a planar compliant electrode being structured and providing one-directional compliance, where metal traces are patterned in parallel lines over a charge distribution layer, both of which cover an active area of a polymer. The metal traces and charge distribution layer are applied to opposite surfaces of the polymer. The charge distribution layer facilitates distribution of charge between metal traces and is compliant. As a result, the structured electrode allows deflection in a compliant direction perpendicular to the parallel metal traces. In general, the charge distribution layer has a conductance greater than the electroactive polymer but less than the metal traces.
The polymer may be pre-strained in one or more directions. Pre-strain may be achieved by mechanically stretching a polymer in one or more directions and fixing it to one or more solid members (e.g., rigid plates) while strained. Another technique for maintaining pre-strain includes the use of one or more stiffeners. The stiffeners are long rigid structures placed on a polymer while it is in a pre-strained state, e.g. while it is stretched. The stiffeners maintain the pre-strain along their axis. The stiffeners may be arranged in parallel or according to other configurations in order to achieve directional compliance of the transducer.
Compliant electrodes disclosed in U.S. Pat. No. 6,376,971 may comprise conductive grease, such as carbon grease or silver grease, providing compliance in multiple directions, or the electrodes may comprise carbon fibrils, carbon nanotubes, mixtures of ionically conductive materials or colloidal suspensions. Colloidal suspensions contain submicrometer sized particles, such as graphite, silver and gold, in a liquid vehicle.
The polymer may be a commercially available product such as a commercially available acrylic elastomer film. It may be a film produced by casting, dipping, spin coating or spraying.
Textured electrodes known in the prior art may, alternatively, be patterned photolithographically. In this case, a photoresist is deposited on a pre-strained polymer and patterned using a mask. Plasma etching may remove portions of the electroactive polymer not protected by the mask in a desired pattern. The mask may be subsequently removed by a suitable wet etch. The active surfaces of the polymer may then be covered with the thin layer of gold deposited by sputtering, for example.
A rolled electroactive polymer device is described in U.S. Pat. No. 6,891,317, where it includes a rolled electroactive polymer and at least two electrodes to provide the mechanical/electrical energy conversion. The rolled electroactive polymer device may employ a mechanism, such as a spring, that provides a force to pre-strain the polymer.
Thus, U.S. Pat. No. 6,891,317 discloses a rolled electroactive polymer, e.g. an electroactive polymer with, or without electrodes, wrapped round and round onto itself (e.g. like a poster) or wrapped around another object (e.g. a spring). The polymer may be wound repeatedly and at the very least comprises an outer layer portion of the polymer overlapping at least an inner layer portion of the polymer. For single electroactive polymer layer construction, a rolled electroactive polymer may comprise between about 2 and about 200 layers. In this case, a layer refers to the number of polymer films or sheets encountered in a radial cross-section of a rolled polymer. The spring provides forces that result in both circumferential and axial pre-strain being applied to the polymer.
Rolled electroactive polymers may employ a multilayer structure, where multiple polymer layers are disposed on each other before rolling or winding. For example, a second electroactive polymer layer, without electrodes patterned thereon, may be disposed on an electroactive polymer having electrodes patterned on both surfaces. The electrode immediately between the two polymers services both polymer surfaces in immediate contact. After rolling, the electrode on the bottom surface of the electroded polymer then contacts the top surface of the non-electroded polymer. In this manner, the second electroactive polymer with no electrodes patterned thereon uses the two electrodes on the first electroded polymer.
Other multilayer constructions are possible. For example, a multilayer construction may comprise any even number of polymer layers in which the odd number polymer layers are electroded and the even number polymer layers are not. The upper surface of the top non-electroded polymer then relies on the electrode on the bottom of the stack after rolling. Multilayer constructions having 2, 4, 6, 8, etc., layers are possible using this technique. In some cases, the number of layers used in a multilayer construction may be limited by the dimensions of the roll and thickness of polymer layers. As the roll radius decreases, the number of permissible layers typically decreases as well. Regardless of the number of layers used, the rolled transducer is configured such that a given polarity electrode does not touch an electrode of opposite polarity. Multiple layers may each be individually electroded and every other polymer layer may be flipped before rolling, such that electrodes in contact with each other after rolling are of a similar voltage or polarity.
Producing electroactive polymers, and in particular rolled transducers, using the technique described in U.S. Pat. No. 6,376,971 and U.S. Pat. No. 6,891,317 has the disadvantage that direction of compliance of the corrugated electrodes is very difficult to control.
Furthermore, it is desirable to make the transducer by rolling a web of a dielectric material, e.g. a polymeric web, having electrodes arranged thereon, since reliability of the device is thereby improved, and a larger actuation force is obtained as the number of windings of the rolled structure increases. Thus, if a large actuation force is desired, then a large number of windings should be provided. However, using the prior art techniques described in U.S. Pat. No. 6,376,971 and U.S. Pat. No. 6,891,317, the possible number of winding is limited due to the necessary minimum thickness of the polymeric film.
Finally, in order to obtain the necessary compliance using the prior art technology, it is necessary to use materials having a relatively high electrical resistance for the electrodes. Since a rolled transducer with a large number of windings will implicitly have very long electrodes, the total electrical resistance for the electrodes will be very high. The response time for a transducer of this kind is given by τ=R·C, where R is the total electrical resistance of the electrodes and C is the capacitance of the capacitor. Thus, a high total electrical resistance results in a very long response time for the transducer. Thus, in order to obtain an acceptable response time, the number of windings must be limited, and thereby the actuation force is also limited, i.e. response time and actuation force must be balanced when the transducer is designed.
Thus, traditional designs of electroactive polymer actuators based on the dielectric polymer principle have so far been on four different configurations, i.e. single actuator sheets of limited dimensions, stacks of single actuator sheets of limited dimensions and numbers, rolled single actuator sheets of limited dimensions and very limited number of windings, and rolled stacks of single actuator sheets of limited dimensions and very limited number of windings.
All of the actuator configurations listed above have a common working principle that they have to be operated in a pre-strained mode either by pre-loading with a mass (constant load, typical for vertical displacements) or by pre-loading with a spring or spring-like elements under compression or in a stretched state. Both of these pre-strained configurations aim at compensating for the lack of stiffness of these very soft electroactive material structures in the direction of actuation. Indeed, when electrical energy is supplied to them, it is the mass or the spring that is delivering most of the mechanical work. The electroactive material delivers work only when electrical energy is removed from it, i.e. it is discharged, due to the restitution of potential energy stored in the electroactive material. In the absence of pre-loading, such electroactive actuators tend to bend or buckle, due to tensile stress, rather than exert any substantial axial actuation force when electrical energy is supplied to them.